This invention relates to a sound reproduction apparatus used in various audio sound systems. Conventionally, a negative feedback circuit has been incorporated in the sound reproduction apparatus in order to secure a stable and high S/N ratio over a wide frequency range improving the acoustic characteristics of speaker.
The sound reproduction apparatus with a conventional negative feedback circuit has been constructed as shown in FIG. 19 where the signal is inputted into input terminal 1 through the positive input terminal of subtracter 2, the output signal of this is amplified by power amplifier 3, and is supplied to speaker 5 disposed in closed cabinet 4. Then, a negative feedback loop is constructed by detecting the acoustic signal radiated from speaker 5 by microphone 6, by amplifying the detected signal by microphone amplifier 7, and by connecting this output signal to the negative input terminal of subtracter 2 through filter 8.
However, with the negative feedback circuit of conventional construction, a phase difference between the electrical signal inputted in speaker 5 and the acoustic signal radiated from speaker 5 is changed as shown in FIG. 20 producing a negative feedback near the phase difference of 0 degree while producing a positive feedback near the phase difference of +180 or -180 degrees producing a problem of unstable and inadequate feedback.
Furthermore, when the bandwidth is limited by filter 8 to obtain a suitable margin for oscillation, the improvement of acoustic characteristics by the application of negative feedback can be obtained only in a limited frequency band so that the improvements of acoustic characteristics can not be obtained well.
Furthermore, since the phase angle at the lowest resonant frequency of speaker 5 is +90.degree. which is in a boundary between the positive and the negative feedback, no improvement of the acoustic output signal in the lowest resonant frequency range could be obtained. Moreover, when microphone 6 detecting the acoustic signal radiated from speaker 5 is disposed in front of speaker 5 as shown in FIG. 19, a problem of detecting not only the acoustic signal radiated from speaker 5 but the external acoustic signals is produced, resulting an inadequate S/N ratio.